[SOLVED] chmod +w /var/log/messages fails for root user? Why?

PaxToYou · 10-05-2016, 03:53 PM

Does anybody know why the root user can add the read right for everybody to the /var/log/messages giving the commands:

$ ls -l /var/log/messages
-rw------- 1 root root 161613 Aug 8 14:12 /var/log/messages

$ chmod +r /var/log/messages

$ ls -l /var/log/messages
-rw-r--r-- 1 root root 161613 Aug 8 14:20 /var/log/messages

but with the same command can not add the write right

$ chmod +w /var/log/messages

$ ls -l /var/log/messages
-rw-r--r-- 1 root root 161613 Aug 8 14:20 /var/log/messages

Thank-you

-----------------
I've solved the problem with a 2 step procedure:
1) editing the /etc/syslog-ng/syslog-ng.conf file and adding into the option {….} block the line perm(0666);
2) rebooting the computer

bathory · 10-06-2016, 02:43 AM

Hi and welcome to LQ,

From the chmod manpage:

Quote:

A combination of the letters ugoa controls which users' access to the file will be changed: the user who owns it (u), other users in the file's group (g), other users not in the file's group (o), or all users (a). If none of these are given, the effect is as if (a) were given, but bits that are set in the umask are not affected.

The (default) umask for file creation is 0022, meaning that the write bit (w) is set for group and others so chmod will ignore it according to the above.

If you are sure you want to change the file, use either one of the following:

Code:

chmod a+w /var/log/messages
chmod 666 /var/log/messages

Regards

c0wb0y · 10-06-2016, 04:35 PM

Not sure if it's even a wise idea to let ordinary users be able to write to system log a la tmp directory?

Habitual · 10-09-2016, 01:52 PM

http://xyproblem.info/

https://www.google.com/#q=sudo%20you...g%20it%20wrong

computersavvy · 11-30-2016, 10:06 PM

I don't believe it is AT ALL WISE to allow ordinary users write permissions on a system log file. It can then be altered at will, possibly maliciously, and even benign changes to it by a user might totally break new valid system entries.

That said, logrotate will likely change its permissions back to the default each time it runs.